Pipelines are the most economically viable method of transporting fluid assets, most commonly oil and gas, but other types of pipeline also exist. A vast pipeline infrastructure exists today responsible for gathering, transporting and distributing these natural resources, with over three quarters of a million kilometers of oil and gas pipelines in the US alone. The continuing proper operation of these pipelines is of paramount importance, and failures carry massive economic loss, environmental impact and potentially catastrophic physical damage also.
Significant efforts are therefore made to maintain, monitor and inspect pipelines. The sheer size of many pipeline networks however, and the fact that many kilometres of pipelines consist of underground or sub-sea installations makes effective and efficient monitoring a difficult problem.
To monitor, inspect and maintain pipelines use is made of various inspection and maintenance tools which are inserted into the pipeline and carried through the pipeline by the pressure of the fluid therein. There are a variety of different objects, commonly referred to as “pigs” that may be employed. A simple cleaning pig may comprise an object shaped to brush or scrape the inner walls of the pipeline as it passes to provide a cleaning action. An intelligent monitoring pig may be comprise various sensors and an onboard processor and can be a very expensive, very complex instrument.
Pigs are also commonly used during the commissioning of a pipeline, for instance to remove debris from the inside of the pipeline and to dewater the inside of the pipeline.
Use of a pig for inspection or cleaning is commonly referred to as pigging. Pigging is often carried out with largely uninterrupted fluid flow through the pipeline and thus is advantageous in that pipeline flow need not be stopped to perform routine inspection and maintenance. However, occasionally pigs may become stuck in a pipeline. This can occur for many reasons, for example a cleaning pig may scrape the walls of the pipeline to clean any material there from. In an oil pipeline for example build up of various hydrocarbon products such as waxy or semi-solid residues can occur. An area of excessive residue build up may present too much of an obstacle to a pig. Also, in normal operation the debris from the cleaning action builds up in front of the pig and in some instances can block passage of the pig. Damage to the pipeline, such as denting or buckling, may also cause a change in the inner dimensions preventing ease of passage of the pig.
A pig which becomes stuck in the pipeline obviously causes severe disruption to the pipeline. The pig may block some or all of the flow and may result in the pipeline being shut down. Obviously it is desirable to clear such a blockage as soon as possible. Pipeline downtime can have significant cost implications and the consequential effects of a failure of supply can be significant in some applications. In addition, as mentioned previously, some inspection pigs are very expensive and there can be cost implications in the pig itself being inoperative.
To clear the blockage it may be necessary to actually cut into the pipeline and remove the offending pig. Clearly however there is a need to detect that the pig has become stuck. The fact a pig is causing a blockage may be detectable by flow monitoring apparatus but it may not be possible to determine whether the pig is still moving, albeit slowly, or whether the pig has stopped moving. For a simple cleaning type pig, the typical pigging run may simply involve inserting the pig at a pig insertion point in the pipeline and later extracting it from a downstream extraction point a certain time later. Detection that the pig has become stuck may only occur when the pig fails to appear at the extraction point within a certain time window after it was inserted. Further, when it is determined that a pig is stuck the actual location of the pig in the pipeline needs to be identified. For a pig without any sensors or transmitters locating the blockage can represent a challenge given that the pipeline may be buried and may be many kilometres long.
It is therefore an object of the present invention to provide methods and apparatus for tracking and/or locating objects moving in a conduit.